The measurement of levels of fill, particularly of fluids including liquids, gases and fluid multi-phase materials such as emulsions and slurries has been carried out for many years, using nucleonic level gauges, by measuring the amount of radiation emitted by a radiation-source which is detected at one or more levels within the vessel. The radiation is attenuated as it passes through materials; the amount of attenuation being related to the density of the materials between the source and a detector. From comparing the attenuation of radiation detected at different levels of the vessel, it is possible to estimate the height of materials contained in the vessel. Such gauges often comprise a source of gamma radiation of sufficient energy to penetrate the vessel walls, if required, and to traverse the material in the vessel as far as the detector(s). The detectors may comprise one or more Geiger-Müller (GM) tubes or a scintillation detector. For example U.S. Pat. No. 3,654,458 describes the detection and control of a liquid level in a sub-sea vessel using a source of ionising radiation and a plurality of detectors.
In typical level gauges, the detector or detectors are arrayed along a linear axis, usually contained within a radiation-permeable casing, and deployed so that the detector array extends generally vertically with respect to the vessel so that, in operation, the level to be measured is located between the first and last detectors of the array. When a single elongate scintillation detector is used it is deployed in a similar fashion so that the level to be measured typically is located between the ends of the scintillator. These typical prior art arrangements are illustrated schematically in FIG. 1. The detectors are controlled and data is collected using electronic data processing and control apparatus which is usually housed in a housing which is of relatively large cross section compared with that of the detector array and normally located at one end of the detector. In the case of a scintillation detector, the housing also contains a photomultiplier, and a voltage converter to produce the high voltages required to operate the photomultiplier. The invention provides an improved detector apparatus for use in a nucleonic level gauge.